1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus, which is capable of, by focusing light onto a light reflecting layer or a recording layer in an information recording medium comprising a transparent substrate or a transparent protective layer and a single or plural light reflecting layers or recording layers from the transparent substrate or the transparent protective layer, reproducing information recorded in the light reflecting layer or the recording layer or recording information in the information recording medium, and further having a correcting mechanism capable of detecting a thickness unevenness from light incident side for use in reproducing or recording, stretched from the surface of the transparent protective layer or the transparent substrate to the light reflecting layer or the recording layer and a correcting mechanism capable of correcting an influence of the detected thickness unevenness, and a method for correcting the thickness unevenness employed therein, and more particularly to,    1) A method for correcting a thickness unevenness by the time when information is reproduced or recording of information is started, after an information recording medium is loaded on the information recording/reproducing apparatus, and startup of control of that method;    2) A method for correcting an influence of thickness unevenness by the time when information is reproduced or recording of the information is started, just after a layer, which light is converged, of the light reflecting layer or the recording layer is moved (the layer is switched) with respect to a recording medium having plural light reflecting layers or recording layers, and startup of control of that method, and    3) A method for detecting a jump between layers (irregular shift of light converging spot between layers) generated at random, when information is reproduced or information is recorded by converging light to any layer in an information recording medium having plural light reflecting layers or recording layers.
2. Description of the Related Art
Jpn. Pat. Appl. KOKAI Publication No. 2000-171346 has disclosed an example of detecting a defocus of an objective lens by means of a defocus detecting system for detecting the defocus according to a knife edge method, and detecting a spherical aberration or a thickness unevenness in a substrate of a transparent recording medium with a single photo detector 7.
In the aforementioned detecting optical system, a hologram 2 used for dividing a light converging spot including the optical axis center into two sections extracts half of light from the center, and separates it to light 2a near the optical axis center and light 2b far from the optical axis center. Then, a light converging spot P1 of the light 2a near the separated optical axis center and a light converging spot P2 of the light 2b far from the optical axis center are detected on division border lines of split detectors 7a-7b and 7c-7d disposed at focusing positions with respect to a photomagnetic disk 6 upon focusing.
As for the signal detection method, this publication indicates    i) detecting a difference in detecting signals from any one (7a-7b or 7c-7d) of the 2-split detectors making a pair as a defocus detecting signal,    ii) calculating a difference in differential value of the detection signals from the 2-split detectors (7a-7b and 7c-7d) making a pair so as to detect a spherical aberration.
Generally, in the information recording/reproducing apparatus (optical disk drive unit) including the aforementioned example disclosed in the Jpn. Pat. Application No. 2000-171346, the spot size D (diameter) of a light converging spot to be irradiated to the recording layer or the light reflecting layer of the information recording medium (optical disk) in order to record information in the recording medium or reproduce information therefrom has such a relation of D∝λ/NA between a numerical aperture NA of the objective lens and wavelength λ of light.
Because the recording density of information to be recorded in the information recording medium depends upon this spot size D largely, this spot size D needs to be reduced in order to improve the recording density.
In a widely prevailing CD type disk, the NA of the objective lens is substantially 0.47 and the wavelength of light for use is λ=780 nm.
In the DVD type disk, the NA of objective lens is substantially 0.60 and the wavelength of light for use is λ=650 nm. Currently, it has been proposed to obtain a recording density several times the DVD type by employing an objective lens whose NA is about 0.9 and light of wavelength of about λ=400 nm to meet a demand for higher density.
In the current CD type and DVD type disks, when converging light on the light reflecting layer or the recording layer, light is irradiated from the side of the substrate or supporting body (beyond the substrate). If it is intended to converge light on the light reflecting layer or the recording layer by intensifying the NA of the objective lens and shortening the wavelength of light, light is irradiated from the side of a covering layer (transparent protective layer) which functions as a protective film for the light reflecting layer or the recording layer provided opposite to the substrate (supporting body).
However, if a thickness unevenness occurs in the thickness of the transparent protective layer, spherical aberration occurs so that light spot converged on the light reflecting layer or the recording layer is expanded thereby providing a problem that recording or reproduction characteristic deteriorates. Meanwhile, the amount of thickness unevenness in the transparent protective layer or the amount of the spherical aberration with respect to the thickness unevenness increases proportional to the fourth power of the NA of the objective lens.
Further, to increase the recording capacity of the information recording medium, it has been already proposed to provide the DVD type disk with two layers of the recording layers or the light reflecting layers and converge light to only any one layer of the respective layers in the same direction. By providing the recording layer or the light reflecting layer with two layers each (or more), a distance from the transparent protective layer differs depending on each layer. Thus, there is generated such a problem that by using an objective lens with a higher NA than 0.6 (for DVD-disk), the degree of spherical aberration generated by the thickness unevenness exceeds its allowance largely in all the layers or some layer exceeding its allowance may occur.
For the reason, there exists a necessity of measuring a thickness unevenness in the transparent protective layer or a substantial thickness unevenness generated in the light reflecting layer or the recording layer so as to correct the spherical aberration in a real time.
Meanwhile, the aforementioned Jpn. Pat. Appl. KOKAI Publication No. 2000-171346 has not described anything about a correction method (control method) for removing an influence of the thickness unevenness based on the detected spherical aberration.
Further, if there are two or more recording layers or light reflecting layers, the light converging spot generated at random often may be moved to a different recording layer or light reflecting layer due to disturbance such as vibration (hereinafter referred to as abnormal jump between the recording layers), thereby leading to difficulty of focus control (generating defocus).